Further to that last post. The pot is wired as a variable resistor, from
0r to 7k (as opposed to a voltage divider). If the pot has gone open
circuit, the circuit isn't going to be too happy. If it measures OS, you
could try soldering some intermediate value resistor (anything you have
between 0r to 7k) and see if the circuit works, albeit with fixed gain.
Another thing that I have found over the years is that a lot of the
time, the weirdest problems turn out to be cracked or broken traces on
the circuit board. Very often the crack is right at the edge of a solder
pad, where it meets the solder-mask (dark green coating over the board).
OTOH, not all circuit boards, especially older ones have solder mask.
Still, I have lost count of how many times I have decided to try and
change just one more part, only to find that as I desoldered it, a whole
pad came up with it, thereby showing where the broken trace was.
Good luck,
Ike
On 2/6/2010 12:06 PM, Ike wrote:
I would take a close look at the gain pot. Check the solder joints,
but also measure the pot with a DMM.
Ike
Sent from my iPhone
On 2010-02-05, at 8:11 PM, Jared <[email protected]> wrote:
Hi everyone
Ok I finally had some time to look at my misbehaving mic pre, and now
I'm even more confused.
So after digging in a bit deeper this is what I've discovered.
Fist problem distorted noise with no input (peak light flashing)
replacing the TL074 seemed to make a slight improvement. But the
signal was still distorted but not as bad. So I reflowed some solder,
I replaced the 100 ohm resistors feeding the chip, and the ones
feeding the transistors, no change. Then I swapped the transistors
from a spare channel no change. This was about the time I realized
that the gain pot was not affecting the signal. It seemed to have
fixed gain (It may have been this way I don't think swapping the
transistors did this). By turning the pot its full range it only mad
a slight change in sound the last 10% of its travel the sound got
darker and that was all that i could hear changing. All of my testing
has been with me talking into a 57 so not very scientific, and I may
not be hearing something more obvious.
Looking at this circuit is confusing me so I think I need someone
with some real knowledge to help guide away from replacing every
component in the mic pre.
I have attached a copy of the schematic.
Sorry for the long post
thanks,
Jared
(612)802-3150
-----Original Message-----
From: Ike Zimbel <[email protected]>
Sent: Jan 26, 2010 2:55 PM
To: [email protected]
Subject: Re: NEOTEK: Mic pre transistors
Hi Matt,
All of the documentation that I have seen says "factory selected"
on the 2SB737's, which are the only one of these transistors that I
have
seen (or at least noticed) in a Neotek mic pre. Incidentally, I have
never seen one fail in a mic pre yet. Jared, I agree with Matt that you
should check all of the other components 1st, both for soldering issues
and value. Have the caps been done?
best,
Ike
On 1/26/2010 12:36 PM, Matt Syson wrote:
Hi Jared
All or some of these transistors are approaching obsolete but should
be obtainable by post from the internet.
I would suggest getting a bag of them (they are not expensive) then
you can match them using a multimeter for similar Hfe.
(note, don't hold them for long as the finger temperature affects
gain!).
You could simply fit one and it will work but distortion would be
reduced by matching. I don't suppose Neotek matched them originally
though.
It may be worth running the multimeter over the whole circuit
comparing it to others (DC conditions) as there may be something else
amiss.
Matt S
Jared wrote:
Hello everyone
I'm having a problem with a series 1 mic pre (just the mic pre, line
in is fine). I'm getting a lot of noise and extremely distorted
signal. Everything looks fine visually. I swapped the chip and that
didn't fix it. My next thought was that it could be caused by a bad
transistor on the front end of the circuit. So I did a little looking
and now I'm a bit confused.
It seems that I need either of these transistors: 2SAB737, 2SA991, or
2SA1084
I'm sure their are others but this is what is in my console and the
2SA1084 was written in some of the documentation that I have. Does
anyone have a lead on where these can be sourced or other replacement
options? Also what is the best way to match them? Can it be a simple
DMM hFE test?
Also maybe I'm going down the wrong path, any other thoughts about
what could be causing this?
Sorry for the long post,
Thanks,
Jared
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Ike Zimbel
Zimbel Audio Productions Ltd.
Toronto
416-720-0887
"Studio House Calls". Authorized Warranty service for Allen& Heath,
Clear-Com, Drawmer, Soundcraft, SPL and Yamaha. Repairs and upgrades
to analog audio equipment including: AMEK, dbx, Neve, Neotek, MCI,
Soundcraft and more. Ask about our RF Frequency co-ordination services!
www.zimbelaudio.com
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<Input.pdf>
--
Ike Zimbel
Zimbel Audio Productions Ltd.
Toronto
416-720-0887
"Studio House Calls". Authorized Warranty service for Allen& Heath, Clear-Com,
Drawmer, Soundcraft, SPL and Yamaha. Repairs and upgrades to analog audio equipment
including: AMEK, dbx, Neve, Neotek, MCI, Soundcraft and more. Ask about our RF Frequency
co-ordination services!
www.zimbelaudio.com
------
View the list online at: http://www.mail-archive.com/[email protected]
You can unsubscribe at anytime by sending an email to mailto:[email protected] with
"unsubscribe neotek" in the message body. If you have any difficulties, or
questions --please send an email to mailto:[email protected].
The Neotek list is lovingly hosted by Catapult Web. Remember that any message
or reply sent to the list will be automatically forwarded to all subscribers.
Vive Le Neotek!
Thanks.
